Field of the Invention
The present invention relates to an organic light emitting display device, and more particularly, to an organic light emitting display device and a driving method thereof, which reduce a size of corrected image data by using a color compression scheme to decrease a capacity of a memory, and reduce a size of an integrated circuit (IC) to increase a manufacturing yield.
Discussion of the Related Art
Recently, organic light emitting devices, which self-emit light without a separate light source and have better viewing angle, brightness, and contrast than those of LCD devices are attracting much interest. Also, since the organic light emitting display devices do not use a backlight, the organic light emitting display device are manufactured to be light and thin, and have low power consumption and a fast response time.
In the organic light emitting display devices, a characteristic of each pixel is changed depending on a driving time and a temperature. Here, a compensation scheme is categorized into an internal compensation scheme and an external compensation scheme depending on a position of a compensation circuit that compensates for a characteristic change of pixels. The internal compensation scheme is a scheme in which a compensation circuit is disposed inside each of the pixels. The external compensation scheme is a scheme in which the compensation circuit is disposed outside each pixel.
Threshold voltages (Vth) and mobility (k) of driving thin film transistors (TFTs) of pixels differ due to a deviation of a process of manufacturing the driving TFTs (DT). For this reason, in general organic light emitting display devices, despite the same data voltage (Vdata) being applied to the driving TFT (DT) of each pixel, a uniform quality of an image cannot be realized due to a deviation of a current flowing in an organic light emitting diode (OLED).
To solve such problems, a threshold voltage change and mobility change of the driving TFT of each pixel are sensed. Subsequently, the threshold voltage change and mobility change of the driving TFT are compensated based on sensing values. Therefore, a driving voltage “k*Vdata+Vth” obtained by summating a data voltage (Vdata) based on an image signal and a compensation voltage (Vth, k) is supplied to a gate of the driving TFT.
FIG. 1 is a diagram illustrating a method of generating corrected image data in a related art organic light emitting display device, and FIG. 2 is a diagram illustrating raw compensation data and after-compression loss-corrected data according to a related art.
Referring to FIGS. 1 and 2, the related art organic light emitting display device loads compensation coefficients for red, green, and blue pixels from an external memory 1, and corrects image data of the red, green, and blue pixels by using the compensation coefficients.
A compression encoder 2 compresses image data which is corrected by a lossy or lossless compression scheme, and the compressed and corrected image data is stored in a static random access memory (SRAM) 3.
A compression decoder 4 loads the corrected image data stored in the SRAM 3, and decodes the compensated image data.
An external compensation unit 5 performs a pixel compensation operation by using the corrected image data to compensate for mura of an OLED panel.
The related art organic light emitting display device uses the same compression encoder and decoder so as to enable the compression encoder and the compression decoder to be easily designed. Generally, a compression of an image is performed by one selected from the lossy compression scheme and the lossless compression scheme depending on a memory size. The lossy compression scheme analyzes a pattern of an image. A method, which removes a high frequency component (which is not perceived by a viewer well) to decrease a size of whole image data, is applied to the related art organic light emitting display device.
In regard to a characteristic of the OLED panel, a compensation coefficient is random data, and thus, when lossy compression is applied to a stored compensation coefficient, a noise component is perceived by a viewer due to a loss error which occurs after compression when a method of reducing a high frequency component is used.
In the lossless compression scheme, a compression rate is greatly lowered, and thus, random data has a compression rate of less than 1.8:1. Therefore, a large-capacity memory of 50 Mbyte or more is needed with respect to a full-HD resolution (1920*1080).
In general image data, a noise component is not easily perceived despite application of the lossy compression scheme. However, in data independent from a screen pattern like a compensation coefficient, a noise component is perceived by applying the lossy compression scheme.
In particular, a mobile display device is limited in size, and for this reason, it is limited to enlarge a capacity of a memory storing compensated image data. On the other hand, when a compression rate of data is low, it is difficult to produce goods.